Composite gas separation membranes and making thereof

ABSTRACT

A composite membrane made of an asymmetric porous support and an ultrathin layer having a thickness of less than 100 nm is provided. A method of making the membrane and method for carrying out gas separations using the membrane are also provided.

FIELD OF THE INVENTION

This invention relates to the field of composite gas separationmembranes. More particularly, this invention relates to composite gasseparation membranes comprising an asymmetric porous support and anultrathin membrane forming polymer film.

BACKGROUND

Much technology has already been developed concerning composite gasseparation membranes. Fundamentally, the purpose for a composite (or"multiple-layer") membrane structure is to allow the selection andcombination of multiple materials which can each perform some of thenecessary functions of the overall membrane better than any one of thematerials could perform all of such functions. The process of selectivepassage of certain types of molecules in gaseous phase through anonporous membrane material is a complex phenomenon occurring on amolecular level. Generally, the molecular selectivity is a combinationof diffusion through the membrane material and gas solubility within themembrane material. Referring to the diffusion concept, the selectivemembrane material must have the very special performance property thatcertain types of molecules preferentially will pass through it,resulting in a concentration of such types of molecules on the permeateside of the membrane. Such selective membrane materials can be veryexpensive to develop and produce, and accordingly they command a highprice. Further, since the gas molecules must physically pass through theselective membrane itself, overall membrane flux will be maximized whenthe selective membrane thickness is minimized. This is a crucialconsideration in designing a gas separation membrane, because higherflux translates into higher productivity. Lower flux directly results ingreater compression requirements to force the gas through the membrane,translating into increased operating costs. As a result of theseconsiderations, the gas molecule selection function of a gas separationmembrane is best performed by an ultrathin layer of aspecially-selected, often expensive selective membrane material.

One class of such membrane forming polymers are those referred to as the"6FDA or 6FDA-type polyimides". These polymers can be formed by (A) thecondensation of5,5'-2,2,2-trifluoro-1-(trifluoromethyl)ethylidene-bis-1,3-isobenzo-furanedione(known as "6FDA") with an aromatic diamine such as1,5-diaminonaphthalene or 1,3-diaminobenzene; and (B) dehydration toyield a 6FDA polyimide. The value and applicability of 6FDA polyimidesas gas separation membranes is well known and documented, e.g., in theHoehn et al U.S. Pat. Reissue No. 30,351 (based on U.S. Pat. No.3,899,309), the Hayes U.S. Pat. No. 4,717,394, and the Ekiner et al U.S.Pat. No. 5,085,676. Other known membrane forming polymers includecellulose triacetate (CTA), polyethersulfone (PES),polytrimethylsilylpropyne (PTMSP), polyetherimide (PEI) andpolypropylene oxide (PPO).

A process for laminating a polymer layer to a paper support is disclosedin the Lundstrom U.S. Pat. No. 3,767,737. In this process a polymersolution is spread out on the surface of a pool of water. When thesolvent of the polymer solution evaporates the polymer solidifies andforms an ultrathin film on the water surface. This thin polymer film isthen picked up by and laminated to a paper web.

Another important function of a gas separation membrane is to withstandthe pressure drop across the membrane which is encountered in andnecessary for its operation, and otherwise endure a reasonable lifetimeas an integral material in the intended operating environment. Thisfunction is best performed by a structural support material which (1)can be prepared economically as a relatively thick layer which willprovide adequate mechanical strength, and (2) is highly. permeable, soas not to markedly reduce the gas flux of the overall membrane.

It was an object of the invention to provide a composite membrane whichwould combine the features of (1) withstanding extensive pressure drops,(2) being highly permeable and (3) having an ultrathin membrane layer,which ultrathin membrane layer would not collapse into the support underoperating conditions.

SUMMARY OF THE INVENTION

Applicants have surprisingly found a composite membrane comprising anasymmetric porous support and an ultrathin polymer layer having athickness of less than 100 nm.

In a further aspect this invention provides a process for making acomposite membrane comprising the following steps:

(A) Forming an asymmetric porous support, such asymmetric porous supportcomprising mutually coplanar first and second regions, said first regionhaving a microporous structure, said second region comprising a firstsurface integrally connected to said first region and a second surface;

(B) Preparing a dissolved polymer solution comprising (1) at least onemembrane forming polymer, and (2) a solvent for the membrane formingpolymer which solvent is immiscible with a selected liquid, saidselected liquid being a nonsolvent for the membrane forming polymer andhaving a higher surface tension than said dissolved polymer solution;

(C) Depositing said dissolved polymer solution onto the surface of apool of the selected liquid, and removing the solvent from the dissolvedpolymer solution to yield an ultrathin film of the membrane formingpolymer; and

(D) Contacting said ultrathin film of membrane forming polymer with saidsecond surface;

yielding a composite membrane comprising an asymmetric porous supportand an ultrathin membrane forming polymer film.

In one embodiment, the composite membrane according to the presentinvention includes a base support which is a porous structural supportmaterial having first and second mutually coplanar surfaces, whichbecomes integrally bonded to the asymmetric porous support when theasymmetric porous support is formed on one of the surfaces of the basesupport in step (A).

In further preferred embodiments, the asymmetric porous support isformed on said planar surface of the support by solvent cast phaseinversion.

The invention also provides a gas separation process comprising the useof such composite gas separation membranes.

Further embodiments will be described below. The various embodimentswill be detailed in the discussion below, and defined in the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a cross-sectional schematic drawing of a composite membrane ofthe invention.

DETAILED DESCRIPTION OF THE INVENTION The Composite Membrane in General

FIG. 1 schematically illustrates a cross-section of a composite membraneof the invention. In this embodiment, the membrane structure includes abase support, which provides mechanical strength to the composite whilenot excessively increasing the pressure drop across the compositemembrane. While being preferred it is, however, understood that suchbase support may not be required and in such cases may not be present.The asymmetric porous support is formed on and integrally bonded to thebase support, if present. Although it is a unitary structure, theasymmetric porous support includes two mutually coplanar, integrallyconnected regions 1 and 2. What we mean by "mutually coplanar" is thateach of the regions is a flat, two-sided, sheet- or web-like material,and that one flat surface of each of them are integrally connectedtogether. The ultrathin membrane forming polymer film is formed onto theasymmetric porous support, adjacent to Region 2. It is a primary goal ofthe invention to provide composite membranes comprising such ultrathinmembrane forming polymer films having a thickness of less than about 100nm. Hence, Region 2 must also have surface pores with a diameter of lessthan about 100 nm, preferably with a diameter of less than about 40 nmand more preferably with a diameter of about 20 nm to about 30 nm.Otherwise, the ultrathin membrane forming polymer film will tend tocollapse into the Region 2 surface pores, both during application of thefilm to the asymmetric porous support, and at operating pressures duringuse. Region 2 must also have a relatively high density of such pores inview of their extremely small size, to avoid excessive pressure dropsacross the membrane. About 2% to about 6% of the surface area of Region2 is constituted by surface pores (measured by Scanning ElectronMicrographs SEM's). Given that Region 2 is designed to have exceedinglysmall-sized pores, Region 2 should also be quite thin in order tominimize its effect in increasing the overall pressure drop across thecomposite membrane. On the other hand, Region 2 must be thick enough sothat it will not collapse into Region 1 during fabrication or underoperating pressures. Hence, Region 2 preferably has a thickness betweenabout 100 nm and about 250 nm. Region 1 is a cushion for Region 2(protecting Region 2 from direct contact with the relatively rough andimperfect surface of the base support), and must balance severalcompeting needs. On the one hand, Region 1 must provide adequate gasflux--for which large pores are preferred. On the other hand, Region 1must provide cushioned support for Region 2, and not itself collapseunder operating pressures--for which small pores are preferred. Tocompromise these contradictory needs, a microporous structure is neededfor Region 1. By "microporous" we mean broadly that the average porediameter in Region 1 is between about 25 microns and about 100 microns,as measured, e.g., by SEM's. The thickness of Region 1 should bebalanced between (1) the role of Region 1 in cushioning Region 2 fromthe base support--for which the thickness should be maximized; and (2)the need to minimize overall membrane pressure drop--for which thethickness should be minimized. Broadly, the thickness of Region 1 shouldbe between about 25 microns and about 100 microns. FIG. 1 alsoillustrates a nonporous protective sealing polymer layer on top of theultrathin membrane forming polymer film. Such nonporous protectivesealing polymer layer, which is optional and preferred, acts to protectthe ultrathin membrane forming polymer film from abrasion, scratches,punctures and the like; and also seals any pinholes which mayunavoidably be present. In particular, where (1) the composite membraneis fabricated in a sufficiently clean environment so that pinholes arenot produced by dust particles deposited on the membrane components, and(2) abrasion of the membrane forming polymer layer is avoided, thenonporous protective sealing polymer layer is optional.

The Base Support

In cases where the base support becomes a permanent base layer for thecomposite membrane, the base support should (1) provide adequatemechanical strength and structural integrity to the overall composite;(2) present a smooth, uniform, planar (flat) surface without protrudingfibers, on which the asymmetric porous support can be formed with aminimum of pinholes and other defects; and (3) contribute a minimalpressure drop to the overall composite. Preferably, the air permeabilityof such a base layer is about 2 to about 5 cm³ /(sec·cm²) of base layer.In preferred embodiments, conventional gas separation membrane supportcloths are employed. Such support cloths are generallyhigh-mechanical-strength porous cloth materials which have a smoothsurface and will not significantly reduce gas flux. Typically, thesuitable support cloths will have a thickness on the order of about 100to about 125 microns. Preferably, woven support cloths made from DACRON®polyester are used. Such woven support cloths tend to have a smooth andflat surface without fiber spikes/defects, which is advantageous fromthe standpoint of applying a smooth and uniform coating thereon. Amongnonwovens, AWA reinforced papers (nonwoven cloth) grade #10, #16-1,#RO26 and #RO27 available commercially from Sanko Limited, Yokohama,Japan are the most preferred materials for use (these grades havedesirably smooth surfaces). Other support cloths that can be usedinclude HOLLYTEX® nonwoven polyester; Type 2430 polypropylene and Type2402 polyester nonwoven cloths from Carl Freudenberg IndustrialProducts; and nylon support cloths.

In cases where it is desired to ultimately separate the base supportfrom the composite membrane, such base support can be any suitabletemporary support for the asymmetric porous support while suchasymmetric porous support is being fabricated (and, if desired, cansimilarly indirectly support further layers of the composite duringfabrication). For example, such base support can be a glass or metalplate; a web (optionally endless) of plastic, metal, or metal laminatedwith plastic such as polyethylene (MYLAR®). Those skilled in the artwill be aware of a multitude of variations of these techniques andmaterials that can be used.

The Asymmetric Porous Support

The porous support for the ultrathin membrane forming polymer isasymmetric. By describing this porous support as asymmetric, it isspecifically meant that the porous support has a thin, dense skinsupported by a thick, porous substrate (matrix) having pores of anaverage size that gradually increases in the direction directly awayfrom the dense skin. Therefore it has one surface area with many smallpores and a second surface area with fewer and bigger pores than in thefirst surface area, yielding a higher porosity in the second surfacearea. By "surface area" we mean a two-dimensional unit surface, such asa cm² or a square inch. Any process yielding the above described type ofstructure can be used to prepare the asymmetric porous support. Inpreferred embodiments, both layers are formed from a single sol by asolvent cast phase inversion process employing a nonsolvent swellingagent. The solvent cast phase inversion process is a general process ofmembrane manufacture that utilizes a sol which inverts into twointerdispersed liquid phases, that is, polymer coated micelles of thedispersed phase in a continuous second liquid phase, prior to, orsimultaneously with gelation and immersion in a liquid precipitatingbath, at which time the emulsoid nature of what once was a sol isimmobilized as a gel.

Solvent cast phase inversion generally is a conventional process whichhas already been the subject of extensive research and teachings byothers. See, e.g., Kneifel U.S. Pat. No. 4,933,085; Kneifel U.S. Pat.No. 4,818,452; Pinnau U.S. Pat. No. 4,902,422; Yamada U.S. Pat. No.4,832,713; Peinemann U.S. Pat. No. 4,673,418; Ekiner U.S. Pat. No.5,085,676; Hayes U.S. Pat. No. 4,944,775; Blume U.S. Pat. No. 5,085,776;Keeling U.S. Pat. No. 4,880,441; Le U.S. Pat. No. 4,853,127; and BehlingU.S. Pat. No. 4,994,094.

To make an asymmetric porous support by solvent cast phase inversion, asuitable polymer is chosen to constitute the asymmetric porous supportmatrix structure, and then a solvent and nonsolvent (swelling agent) areselected. The solvent must dissolve the polymer and be capable ofdissolving the nonsolvent as well. The nonsolvent is required in orderto achieve the necessary pore structure for the asymmetric poroussupport. The nonsolvent must be miscible with the solvent, but must nottotally dissolve the polymer. The nonsolvent must, however, be aneffective swelling agent for the polymer, thus introducing pores in thepolymer. Selection of the solvent and swelling agent should also takeinto account the quenching bath liquid to be used. The structure of theasymmetric porous support is "frozen" by quenching the nascent poroussupport in a precipitating bath liquid. The chosen bath liquid must beimmiscible with the polymer, yet miscible with the solvent and swellingagent. The polymer, solvent and nonsolvent are then mixed in appropriateproportions and spread into a thin film on a support using e.g. a doctorknife. The thickness of the thin film preferably should be between about8 and about 10 μm.

Preferably, for making gas separation membranes, a polyetherimide is thepolymer composition for forming the asymmetric porous support. Thepolyetherimide employed may be a commercially available plastic (e.g.,ULTEM® made by General Electric). These resins are amorphousthermoplastics. ULTEM® grades in the 1000 series (standard viscosity)are preferred, especially the "natural" type which does not containcolorants. Other structurally related polyetherimides can also be usedif they are soluble in one of the solvents mentioned below. Using ULTEM®1000 polyetherimide polymer resin, asymmetric porous supports can beprepared having, in Region 2 (the dense skin), a porosity of about2-10%, preferably 4-8%, especially preferred between about 6.2% andabout 6.6% (based on the total surface area, length-width, of the faceof the asymmetric porous support), the average pore diameter being about2-50 nm, preferably about 10-15 nm, especially preferred in betweenabout 12.8 nm and about 13.8 nm, the number of such pores per unitsurface area being about 10¹⁰ -10¹¹ pores/cm², preferably about 3×10¹⁰-8×10¹⁰ pores/cm², especially preferred in between about 4.3×10¹⁰pores/cm² and about 4.8×10¹⁰ pores/cm² (these parameters can bedetermined by SEM measurements).

The polyetherimides used herein are polymers having a molecular weightof 10,000 to 50,000 and preferably 25,000 to 40,000, and can be preparedby condensation reaction of phenoxy-phenyldicarboxylic acid anhydrides(such as 2,2-bis-4-(3,4-dicarboxyphenoxy) phenyl propane anhydride) andphenylenediamines (such as methaphenylenediamine). In thephenoxyphenyl-dicarboyxlic acid anhydrides, the carboxy and phenoxygroups may be located at 3,3'-4,4'- or 3,4'-positions. In addition, amixture of such 3,3'-, 4,4'- and 3,4'-substituted compounds may be used.Although it is most preferred for the propane to take a --C(CH₃)₂-structure, it may be --CH₂ --CH₂ --CH₂ -- or --CH₂ --CH(CH₃)--. InC_(n) H_(2n) - other than the propane, n may be within the range of 1 to8.

Suitable polyetherimide polymers are disclosed in Ekiner U.S. Pat. No.5,085,676 and Yamada U.S. Pat. No. 4,832,713. Methods for preparingsuitable polyetherimides are disclosed in, for example, Heath et al.U.S. Pat. No. 3,847,867, and Verbicky, Jr. et al U.S. Pat. No.4,774,317.

The polyetherimide can be dissolved in tetrahydrofuran, dioxane,dimethylacetamide, N-methyl-2-pyrrolidone, dimethyl sulfoxide, and othernitrogen-containing cyclic solvents such as N-formylpiperidine and1-formylmorpholine. The nitrogen-containing cyclic solvents exhibit ahigh ability to dissolve the polyetherimide or the polymer mixture, evenup to a concentration of about 30% by weight. Thus, of the solvents asdescribed above, the nitrogen-containing cyclic solvents areparticularly preferred. The polymer concentration determines porosity;if the concentration is too high, the size of the asymmetric poroussupport pores will be too small; if the concentration is too low, thesize of the pores in the asymmetric porous support will be too large.The polymer concentration for a particular polymer-solvent-nonsolventsolution can best be optimized on a case by case basis. Generally,however, the polymer concentration in the solvent should be betweenabout 5 and about 40% by weight, and preferably 15 to 25% by weight.

It is also necessary to add to the above-described solvent or solventmixture a pore-forming substance (swelling agent). The swelling agentshould be a nonsolvent for the polyetherimide. The swelling agent mustnot react with the polyetherimide or with the solvent. Examples ofswelling agents include: acetic acid, toluene, xylene,gamma-butyrolactone, and dimethyl sulfoxide. The swelling agentconcentration in the polymer-solvent-nonsolvent solution may lie between5 and 40 weight percent.

With ever-increasing pressure to reduce the use of chemical solvents inresponse to worker safety and health issues, environmental emissioncontrols, and solvent cost, water is the precipitating agent of choice.All organic liquids which are nonsolvents for polyetherimide but arecompletely miscible with all other components of thepolymer-solvent-nonsolvent solution can be used as membraneprecipitation agents. The precipitation agent can be used to influencethe pore structure and thickness of the dense skin of Region 2. Thequench may also contain organic or inorganic additives to alter thethermodynamic activity of the quench components, thereby altering theinflux rate of the quench components into the nascent asymmetric poroussupport.

Other thermoplastic resins can be used for forming the porous support,although as stated above, polyetherimides are preferred. A second choicefor gas separation membranes is polyacrylonitrile, such as a copolymerof 94% polyacrylonitrile and 6% polymethylacrylate made by DuPont underthe name Polymeric Acrylonitrile Type A-7 ("PAN A-7"), having a weightaverage molecular weight of 100,000, which also contains a minor amountof certain unknown copolymer additives. Polyamides such as a solution of25% by weight in dimethylacetamide of the lithium chloride salt of apolyamide polymer having the structure of: ##STR1## and NOMEX® fromDuPont, may be suitable. UDEL®-3500 polysulfone resin availablecommercially from Amoco Oil Company can be used (the swelling agentoften can be omitted). VICTREX®5200G polyethersulfone resin (viscosity10300 centipoise) and VICTREX® 300P polyethersulfone resin availablecommercially from Imperial Chemicals Inc. - Americas can likewise beused. Other types of polymers that may be effective for particularapplications include polyetherketones and cellulose acetate. Furtherdisclosures of polymers for making asymmetric porous supports are foundin Bikson U.S. Pat. No. 5,067,971; Yamada U.S. Pat. No. 4,832,713; KrausU.S. Pat. No. 4,964,990; Blume U.S. Pat. No. 4,963,165; and Baker U.S.Pat. No. 4,857,080.

The Ultrathin Polymer Membrane

As we have already summarized above, the Hoehn, Hayes and Ekiner patentsall relate to 6FDA-type-polyimide polymers which are polymers which maybe employed in the present invention for forming the ultrathin membranelayer. We hereby incorporate these three patents by reference in theirentirety, and will rely on and refer to their teachings. Further6FDA-type polyimide polymers which can be used to form the ultrathinmembrane forming polymer films of the invention are disclosed in thefollowing U.S. patents: Hayes U.S. Pat. No. 4,707,540; Hayes U.S. Pat.No. 4,717,393; Makino U.S. Pat. No. 4,528,004; Makino U.S. Pat. No.4,690,873; Makino U.S. Pat. No. 4,474,858; Makino U.S. Pat. No.4,440,643; Makino U.S. Pat. No. 4,460,526; Makino U.S. Pat. No.4,474,662; Makino U.S. Pat. No. 4,485,056; Makino U.S. Pat. No.4,512,893; Makino U.S. Pat. No. 4,378,324; Ekiner U.S. Pat. No.5,015,270; Ekiner U.S. Pat. No. 4,983,191; Hayes U.S. Pat. No.4,932,983; Hayes U.S. Pat. No. 5,178,650; Hayes U.S. Pat. No. 5,034,024;Kusuki U.S. Pat. No. 5,141,642; Strathman U.S. Pat. No. 4,071,590; andManwiller U.S. Pat. No. 4,622,384. In sum, the term "6FDA-typepolyimide" for purposes of this invention includes and is constituted bythe above-discussed disclosures of all such polymers included in thesepatents. 6FDA itself is the dianhydride of choice for producing6FDA-type polyimides. (The synthesis of 6FDA itself is disclosed, e.g.,in Scola U.S. Pat. No. 4,569,988).

Further polymers which can be employed as membrane forming polymersaccording to this invention include

cellulose triacetate (CTA) ##STR2## with an average degree ofacetylisation typically in the range of about 2.5 to 3.0;

hexafluoropolysulfones (6F-PS) like ##STR3## polytrimethylsilylpropyne(PTMSP) ##STR4## polysulfones (PS) ##STR5## with R¹ and R² representingbivalent organic radicals; polyethersulfones (PES) ##STR6## with R¹ andR² representing bivalent organic radicals. Examples of suchpolyethersulfones are compounds according to the formulae ##STR7##Further membrane forming polymers include poletherimides (PEI) ##STR8##with R¹ and R² representing bivalent organic radicals; such as ##STR9##Further membrane forming polymers include polycarbonates (PC) ##STR10##with R representing a bivalent organic radical; and polypropylene oxide(PPO) ##STR11## (n in all of the above formulae representing the averagenumber of repetitive units per molecule).

It is essential to provide the membrane forming polymer films in anultrathin form: that is, having a thickness of less than about 100 nm,preferably less than about 40 nm and most preferably about 20 to about30 nm. Although it is not readily feasible to directly measure suchsmall thickness dimensions, they can be measured indirectly. Forexample, a petri dish can be weighed before and after an ultrathinmembrane forming polymer film of measurable dimensions is picked up ontop of the dish. Then, using the formula volume=mass/density, andseparately measuring the density of a large sample of the membraneforming polymer, the thickness of the ultrathin film can be calculated.Alternatively, a known mass of membrane forming polymer can be cast on asurface having a known surface area (length width), and the thicknesscan be calculated using the same formula. It is further essential toprovide the membrane forming polymer films with such low roughness thatthey do not protrude into the pores of the porous support. Evenness orlow roughness is also important for a homogeneous membrane performanceover a given surface area. The ultrathin membrane forming polymer filmsof the composite membranes according to the invention have a roughnessof about ±0.5 to 5 nm, preferably about ±1 to 3 nm, even more preferredof about ±1.5 to 2.5 nm. "Roughness" in this context means that thethickness of the ultrathin film varies with these numbers (i.e.thickness ± about 2 nm).

Some membrane forming polymers usefull in practicing the invention areidentified in TABLE 1 below; our abbreviated designations for thesepolymers are also given. Relative proportions, where given, are molaramounts.

                                      TABLE 1                                     __________________________________________________________________________    DESIGNATION                                                                              MONOMERS/PROPORTIONS                                               __________________________________________________________________________    6FDA-F33   6FDA + 3,3'-DIPHENYLHEXAFLUOROISOPROPYLIDENE DIAMINE               6FDA-6F44  6FDA + 4,4'-DIPHENYLHEXAFLUOROISOPROPYLIDENE DIAMINE               6FDA-DB    6FDA + 1,3-DIAMINOBENZENE                                          6FDA-DUR   6FDA + 2,3,5,6-TETRAMETHYL-1,4-PHENYLENEDIAMINE                    6FDA-NDA   6FDA + 1,5-DIAMINONAPHTHALENE                                      6FDA-ABN   6FDA + 3,5-DIAMINOBENZONITRILE                                     6FDA-BENZOATE                                                                            6FDA + 3,5-DIAMINOMETHYLBENZOATE                                   6FDA-DURS  6FDA + 50% 2,3,5,6-TETRAMETHYL-1,4-PHENYLENEDIAMINE +                         50% 3,3'-DIAMINOPHENYLSULFONE                                      6FDA-DBNDA 6FDA + 50% 1,3-DIAMINOBENZENE + 50% 1,5-DIAMINO-                              NAPHTHALENE                                                        6FDA-3,4 SULFONE                                                                         6FDA + 50% 3,3'-DIAMINOPHENYLSULFONE + 50% 4,4'-DIAMINO-                      PHENYLSULFONE                                                      6FDA-STSN  6FDA + 50% 3,3'-DIAMINOPHENYLSULFONE + 50% 3,7-DIAMINO-                       2,8-DIMETHYLDIPHENYLENESULFONE                                                6FDA + 75% 3,3'-DIAMINOPHENYLSULFONE + 25% 3,7-DIAMINO-            6FDA-3STSN 2,8-DIMETHYLDIPHENYLENESULFONE                                     6FDA-SNDA  6FDA + 50% 3,3'-DIAMINOPHENYLSULFONE + 50% 1,5-DIAMINO-                       NAPHTHALENE                                                        6FDA-DBS   6FDA + 50% 1,3-DIAMINOBENZENE + 50% 3,3'-DIAMINO-                             PHENYLSULFONE                                                      MATRIMID   3,3',4,4'-BENZOPHENONE TETRACARBOXYLIC DIANHYDRIDE +                          5(6)AMINO-1-(4'-AMINOPHENYL)-1,3,3,-TRIMETHYLINDANE                CTA        CELLULOSETRIACETATE WITH A DEGREE OF SUBSTITUTION OF                          2.84 (ACETYL CONTENT: 43.3-43.9 WT %)                              HEXAFLUOROPOLY-                                                                          BIS-(p-CHLORPHENYL)SULFONE + HEXAFLUOROBISPHENOL-A                 SULFONE                                                                       PES        p-PHENOXY-PHENYLSULFONYLCHLORIDE                                   TETRAMETHYL-                                                                             BIS-(p-CHLORPHENYL)SULFONE + TETRAMETHYLMETHYLENE-                 METHYLENE- BISPHENOL                                                          POLYSULFONE                                                                   PTMSP      TRIMETHYLSILYLPROPYNE                                              __________________________________________________________________________

Membranes made with 6FDA-ABN (6FDA+3,5-diaminobenzonitrile) areparticularly effective in separating nitrogen/oxygen mixtures. The6FDA-STSN, 6FDA-3STSN and 6FDA-SNDA polyimides are preferred for use inseparating carbon dioxide from hydrocarbons such as methane, ethane,propane, etc.; and the 6FDA-NDA and 6FDA-ABN polyimides are preferredfor use in carrying out air separations. In addition to these 6FDA-typepolyimides, other polymers made using closely-analogous aromaticdiamines are also preferred for use. For example, the --CN group in3,5-diaminobenzonitrile can be replaced by --CF₃, --COOH, --CI, --Br,--OR --OH. The --COOH type is particularly desirable because of theclose molecular packing which it has due to hydrogen-bonding. Anotherpreferred diamine is 2-carboxy-3,5-diaminotoluene.

The 6FDA-type polyimide polymers are synthesized by conventionaltechniques thoroughly reviewed in the above-listed prior art references.

CTA can be purchased from Eastman Company, Tennessee. It can also bemade from commercially available cellulose by subjecting cellulose to anacetylisation reaction. Typically employed CTA has a degree of acetylsubstitution of about 2.5 to about 3.0 and an acetyl content of about 40to about 44 weight-%.

6F-PS can be obtained from reacting commercially availablebis(p-chlorophenyl)-sulfone with hexafluorobisphenol-A.

PTMSP can be purchased from Air Products Laboratory.

PES especially polyarylethersulfone can be purchased from BASF Corp.under the tradename ULTRASON E6010 Q691₋₋ NATURAL. It can also be madefrom p-phenoxy-phenylsulfonylchloride by condensation reaction. PEI canbe purchased from General Electric Company

Tetramethylmethylenepolysulfone can be obtained by reactingbis-(p-chlorophenyl)-sulfone with tetramethylmethylenebisphenol.

PPO can be purchased from Polysciences.

The Silicone Protecting Layer

The ultrathin membrane forming polymer layer of the composite membranesof the invention is vulnerable as the top, exposed composite membranelayer, to abrasion and tearing as a result of any unintended contactwith foreign materials in handling or use. Therefore, in preferredembodiments, a nonporous protective sealing polymer layer is appliedover the ultrathin membrane forming polymer layer. Any materials taughtin the art as useful for forming a nonporous protective sealing polymerlayer on a gas separation selective membrane can be employed, so long asthe material and the solvent used to dissolve it during coating, do notdissolve or react with the ultrathin membrane forming polymer layer.Particularly preferred for use as nonporous protective sealing polymerlayer are the SYLGARD® silicones (e.g., SYLGARD® 184 and 186, which arethermally-curable polydimethylsiloxanes) available from Dow CorningCorporation (these materials are elastomeric silicones which thermallycure to a nonporous crosslinked silicone materials having highpermeability). The use of these materials as non porous protectivesealing polymer layers and methods for their application are disclosed,for example, in: Henis et al U.S. Pat. No. 4,230,463 (see, e.g., theExamples); and Ekiner U.S. Pat. No. 5,085,676 (see, e.g., the Examples).Another silicone composition that can be used is an Ultraviolet curableantimony-complex (GE 9310C) catalyzed silicone supplied by the GeneralElectric Company. Yet another is a thermally curable platinum-complex(PC-072) catalyzed silicone supplied by Dow Corning Chemical Company.The platinum catalyst PC072 and the cross-linker PS123 (to acceleratecuring) are obtained from the Huls Petrarch Systems.

Optional Gutter Layer

Ideally, the ultrathin membrane forming polymer layer is formed directlyon the asymmetric porous support. The two-layer composite then combinesoptimum selectivity, gas flux and mechanical durability. However, thereare circumstances in which it is desirable to interpose a gutter layerbetween the ultrathin membrane forming polymer layer and the asymmetricporous support, in order to facilitate and improve the combination ofsuch membrane materials. In this regard, reference is made toapplicant's copending U.S. patent application Ser. No. 07/999,449, nowU.S. Pat. No. 5,286,280 entitled "Composite Gas Separation MembraneHaving A Gutter Layer Comprising a Crosslinked Polar Phenyl-ContainingOrganopolysiloxane And Method For Making The Same" (which is herebyincorporated by reference). That application discloses such gutterlayers and processes for their fabrication, which can be employed inconjunction with the present invention if desired.

Combination of Functional Membrane Layers

Optionally, more than one ultrathin membrane forming polymer layer canbe formed on the same asymmetric porous support. If desired, two or moredifferent membrane forming polymers having different gas flux capacitiescan be used. For example, a membrane forming polymer having aparticularly high flux rate can be formed on the asymmetric poroussupport before adding a second layer formed from a membrane formingpolymer having a lower flux. In TABLE 2 below, we provide calculatedfilm thicknesses, based on the measured permeance and the calculatedfluxes of 50 and 20 nm films.

                  TABLE 2                                                         ______________________________________                                        THIN FILM COMPOSITE MEMBRANE THICKNESS                                        CALCULATED FROM THE DENSE FILM                                                        MEMBRANE FORMING                                                                              CALCULATED                                            EXAMPLE POLYMER         THICKNESS                                             ______________________________________                                        1       6FDA-SNDA       20.3 nm                                               3       6FDA-DURS       77.3 nm                                               4       6FDA-DBS        34.7 nm                                               5       6FDA-DBNDA      82.1 nm                                               6       6FDA-3,4SULFONE 47.1 nm                                               7       6FDA-NDA        29.8 nm                                               8       6FDA-DB         52.4 nm                                               10      6FDA-ABN        55.8 nm                                               11      6FDA-BENZOATE   40.4 nm                                               12      6FDA-6F44       96.2 nm                                               13      6FDA-6F33       25.2 nm                                               14      MATRIMID        77.1 nm                                               15      CTA             32.7 nm                                               16      HEXAFLUOROPOLY- 135.6 nm                                                      SULFONE                                                               17      PES             49.9 nm                                               18      TETRAMETHYL-    N/A                                                           METHYLENE-      (because standard values for                                  POLYSULFONE     intrinsic permeability are not                                                available)                                            19      PTMSP           N/A                                                                           (because intrinsic permeability                                               changes due to exposure to                                                    UV radiation)                                         ______________________________________                                    

Process for Making the Membrane

In general the process for making the composite membrane according tothe invention comprises:

providing a pool of a selected bath liquid;

spreading out a solution of the membrane forming polymer onto thesurface of the pool;

removing the solvent from the membrane forming polymer solution,yielding an ultrathin film of the membrane forming polymer floating onthe surface of the pool; and

picking up and depositing this ultrathin film onto the surface of aporous support.

In order to carry out the lamination of the ultrathin membrane formingpolymer film onto the asymmetric porous support, a solvent for themembrane forming polymer and an appropriate bath liquid are selected.Water is the preferred bath liquid from a standpoint of nontoxicity; italso has a desirably high surface tension (about 76 dyne/centimeter).Any other liquid having an adequately high surface tension andcompatible with the composite membrane materials can be used as the bathliquid. Those skilled in the art can identify other materials for use asthe selected bath liquid. We will refer specifically to water as theselected bath liquid. This bath liquid is then provided in a pool.

The membrane forming polymer must be dissolved in a suitable solvent.There are two requirements that must both be satisfied by the chosensolvent: (1) the resulting solution of membrane forming polymer in thesolvent must have a lower surface tension than the selected bath liquid(water); and (2) the solvent must be immiscible with the selected bathliquid (water). Suitable solvents that pass both of these tests includethe following: a mixture of 1,2,3-trichloropropane (10-80 wt %) andmethylene chloride (90-20 wt %); a mixture of 90-50 wt % dichloromethaneand 10-50 wt % 1,2,3-trichloropropane; chloroform; a mixture of 90-50 wt% chloroform and 10-50 wt % 1,2,3-trichloropropane; a mixture of 90-50%dichloroethane and 10-50 wt % 1,2,3-trichloropropane;1,2-dichloroethane; other halogenated hydrocarbons (which generally havea low surface tension, e.g., about. 20 dyne/centimeter); a mixture of1,2,3-trichloropropane and ortho-dichlorobenzene (1:1 to 10:1 volumetricproportions); butyl acetate; propyl acetate; toluene; andtetrahydrofuran. Butyl acetate is most effective in solubilizing6FDA-type polyimide polymers in which both the diamine and thedianhydride used to synthesize the 6FDA-type polyimide polymer have F₃C+CF₃ ("6F") moieties. Other generally suitable solvents having a higherboiling point than 1,2,3-trichloropropane include chloroform (60.9° C.),and 1,2-dichloroethane (83° C.). The solvent system used must evaporateslowly enough so that the membrane forming polymer can spread into anultrathin film on the surface of the water pool, and quickly enough sothat the membrane forming polymer film has solidified by the time itreaches the point where it is picked up.

A current is generated in the water pool so that the spread out polymersolution is carried away from its point of contact with the water pooltowards a point where the solidified polymer film is picked up by theporous support. The speed of the surface of the water bath is adjustedto be slow enough so that the polymer solution has enough time tosolidify into a solid film before reaching the pickup point, and fastenough so that the polymer solution being deposited on the surface ofthe water pool does not cover previously deposited solution--whichresults in uneven polymer film thickness.

From a reservoir (roll) the asymmetric porous support is continuouslyfed towards a pickup point, where it contacts the ultrathin polymerfilm. After contacting the ultrathin polymer film, which results in alamination of both films (porous support+ultrathin film), the laminateis carried away from the water pool and subsequently wound up. The speedof the asymmetric porous support web as it passes by the pickup point isregulated to match the speed of advance of the membrane forming polymerfilm on the surface of the water pool, so that the asymmetric poroussupport and membrane forming polymer film join together at the pickuppoint without crinkling or tearing of the membrane forming polymer film.It is preferred that the asymmetric porous support picks up the membraneforming polymer film from below the surface of the water pool so thatthere is less danger of entrapping air in between the two layers.Optionally, the newly-formed composite membrane can be directed over avacuum device such as a vacuum roller or ramp to remove any residuals ofwater and solvents.

Utility

The composite membranes of this invention can be applied to a widevariety of gas separation end uses. For example, the composite membranescan be used to separate methane from nitrogen, carbon dioxide and/orhelium. The composite membranes can also be used for the separation ofoxygen from air to provide enriched oxygen to provide enhancedcombustion; for the separation of nitrogen from air to provide inertingsystems; for the separation of hydrogen and/or nitrogen from ammonia; inthe recovery of hydrogen from hydrocarbon gas in refinery plants; forthe separation of carbon monoxide from hydrogen in syngas systems; forthe separation of carbon dioxide or hydrogen sulfide from hydrocarbons;and for the separation of helium from air, oxygen or nitrogen. Further,the membranes of the invention may have application outside gasseparation, e.g., in microfiltration, reverse osmosis, pervaporation,vapor separation, organic vapor dehydration, and dehumidification.

The composite gas separation membranes of the invention can beconfigured in a variety of types of modules. The various types ofmodules can then further be incorporated, as well known by those skilledin the art, into "systems", by which we mean, e.g., skid-mounted turnkeyapparatus including modules, instrumentation, pressure-, temperature-,and flow-controlling equipment, piping and other components into whichfeed gas to be processed is piped in from its source.

EXAMPLES

Before reporting on the experiments below, we provide some neededterminology.

When a gas species passes through a membrane, the flow rate Q isproportional to the area A of the membrane, and the pressure differenceΔp across the membrane, but reciprocally proportional to the thickness Lof the membrane. That is,

    Q=(P·A·Δp)/L

where P is a characteristic of the membrane, the proportion constantnamed "permeability". The permeability P of the membrane varies with thegas species and the operation conditions such as temperature, pressure,and the gas composition if gas mixtures are used. The thickness L of themembrane usually is difficult to define or measure when the membrane isin asymmetric or ultrathin form, the latter of which is only a couple ofnm to about 200 nm thick. Therefore, in practice, the ratio P/L ofpermeability to thickness, which is hereinafter called "permeance" isused as a measure of the performance of the membrane. That is, accordingto the previous equation, ##EQU1## where Q, A and Δp are easilymeasurable. In this formula the units we use are:

    ______________________________________                                        Q:  standard cubic feet per hour, or SCCS!                                                         (std. cm.sup.3 /sec or SCCS)                             A:  ft.sup.2 !       (929 cm.sup.2)                                           Δp: (100 psi)  (517 cm Hg)                                              P/L:  SCFH/ft.sup.2 /100 psi!                                                                      (6.1 · 10.sup.-6 cm.sup.3 (STP)/                                     cm.sup.2 · sec · cm ·         ______________________________________                                                             Hg)                                                  

The selectivity α of a membrane (also called separation factor) inseparating two gas species A and B from each other is defined as theratio of their gas permeances in that membrane. That is, ##EQU2## Theselectivity α(A/B) can be obtained by measuring the gas permeance ofeach gas in pure gas state or in mixed gas state. The selectivity is ameasure of how well the membrane can separate the gases while thepermeance is a measure of how fast the membrane can let the gases passthrough. In most applications, the membrane is made to maximize bothpermeance and selectivity.

Preparation of Asymmetric Porous Supports

The polymer used to fabricate the asymmetric porous supports for theEXAMPLES below was a polyetherimide supplied by General Electric Companyunder the trade name Ultem® 1000, referred to as Ultem® hereinafter. Foreach EXAMPLE, a 15 to 20% by weight (preferably between 18 and 19% byweight) Ultem® casting solution was prepared by dissolving the polymerin equal amounts by weight of dimethylsulfoxide (DMSO) and1-methyl-2-pyrrolidinone (NMP) solvents at about 80 to 90° C. The Ultem®1000 has an average molecular weight of about 32,000 grams/mole. Thesolvents for making the asymmetric porous support were reagent gradesolvents purchased from Fisher Scientific Corporation.

The Ultem® casting solution was first filtered through a 5 micron filterand then subjected to vacuum for about 5 hours in order to eliminate airbubbles. The resulting dope solution was casted on a smooth DACRON®polyester backing in a thickness of 76 μm to 89 μm by means of a doctorknife. The nascent solution film was immediately immersed in water atroom temperature to coagulate the polymer and to wash out both DMSO andNMP solvents. The thus formed membrane was carefully washed with waterto remove residual solvent and dried by passing it through an oven at50° to 60° C.

ULTRATHIN MEMBRANE FORMING POLYMER FILM PREPARATION

Materials:

5,5'-2,2,2-trifluoro-1-(trifluoromethyl)ethylidene-bis-1,3-isobenzofuranedione (6FDA) was obtained from HoechstCelanese Corporation (commercially available) and was used as received.3,3'-diaminophenylsulfone, 3,7-diamino-2,8-dimethyldiphenylenesulfone,and 4,4-diaminophenylsulfone were purchased from Ken Seika Corporation(respectively designated by Ken Seika as 3,3-DDS; 4,4-DDS; and TSN) andused as received. 1,3-diaminobenzene,2,3,5,6-tetramethyl-1,4-phenylenediamine, 3,5-diaminobenzoic acid,3,5-dinitrobenzonitrile and 1,5-diaminonaphthalene were supplied byAldrich Chemical Company; they were recrystallized before being used.3,5-diaminomethylbenzoate and the 3,5-diaminobenzonitrile weresynthesized from 3,5-diaminobenzoic acid (commercially available fromAldrich Chemical Company; esterified to form the benzoate) and3,5-dinitrobenzonitrile (reduced to the diamino form) respectively. CTApurchased from Eastman Company, Tennessee, designated CA435-75. It had adegree of acetyl substitution of 2.84 and an acetyl content of 43.3 to43.9 weight-%. Polyphenylethersulfone was purchased from BASF Corp.Parsippany, N.J., designated ULTRASON® E6010 Q691₋₋ NATURAL. PTMSP waspurchased from Air Products Laboratory, Allentown, Pa. The solvents usedin synthesizing the polymers and for the process of making the ultrathinmembrane forming polymer films were high performance liquidchromatography grades purchased from Aldrich Chemical Company,Milwaukee, Wis. The MATRIMID polyimide is sold under the trade nameAraldite® XU 218 by the Ciba-Geigy Corporation.

The Plyimide Polymers Were Prepared as Follows:

The 6FDA-type-polyimides employed in these examples were eithercondensation polymers of 6FDA and a diamine or condensation copolymersof 6FDA and a diamine mixture (the MATRIMID example (#14) being anexception), because there the 6FDA is replaced by3,3',4,4'-benzophenone-tetracarboxylic dianhydride. The reactionpartners for forming the polyimides of the EXAMPLES are reported inTABLE 3, below. In the preparation of 6FDA-type polyimides 100 mole-% ofthe diamine or corresponding diamine mixture (typically a 50:50 mole-%mixture) was first dissolved in N,N-dimethylacetamide (DMA). To this wasadded 100 mole-% (stoichiometric amount) of 6FDA. After about 24 hoursof tumbling the mixture in a closed container at ambient temperature,the resulting viscous polyamic acid solution was then dehydrated byreaction with acetic acid anhydride in the presence of pyridine at 80 to100° C. for one hour. The resulting polyimide solution was then slowlyprecipitated in a large excess of methanol under vigorous agitation. Thepolyimide was then recovered by filtration, washed twice with methanoland dried under vacuum, first for 16 hours at room temperature and thenfor 5 hours at 100° C.

The resulting 6FDA-type-polyimide was then dissolved in a solventmixture containing 75% by weight of dichloromethane (DCM) and 25% byweight of 1,2,3-trichloropropane (TCP), degassed, and filtered through a0.5 micron Teflon® membrane supplied by Millipore Corporation beforebeing used.

Preparation of Hexafluorpolysulfone and Tetramethylmethylenepolysulfone

Hexafluorpolysulfone was synthesized from bis(p-chlorphenyl)-sulfone andhexafluorobisphenol-A. Tetramethylmethylenepolysulfone was synthesizedfrom bis(p-chlorphenyl)-sulfone and tetramethylmethylenebisphenol.

TABLE 4 lists EXAMPLES 15 to 19 with their respective chemical formulae.Index "n" in these formulae indicates the average number of repetitiveunits per molecule.

                                      TABLE 3                                     __________________________________________________________________________    FORMULATIONS FOR CONDENSATION REACTIONS YIELDING 6FDA-POLYIMIDE               EXAMPLE - NAME                                                                           6FDA  DIAMINE I      AMOUNT                                                                              DIAMINE II          AMOUNT              __________________________________________________________________________    (1) 6FDA-SNDA                                                                            100 mole %                                                                          3,3'Diaminophenylsulfone                                                                      50 mole %                                                                          1,5 Diaminonaphthalene                                                                            50 mole %           (2) 6FDA-STSN                                                                            100 mole %                                                                          3,3'Diaminophenylsulfone                                                                      50 mole %                                                                          3,7 Diamino-2,8 Dimethyldiphenylenes                                          ulfone              50 mole %           (3) 6FDA-DURS                                                                            100 mole %                                                                          3,3'Diaminophenylsulfone                                                                      50 mole %                                                                          2,3,5,6-tetramethyl-1,4-phenylenedia                                          mine                50 mole %           (4) 6FDA-DBS                                                                             100 mole %                                                                          3,3'Diaminophenylsulfone                                                                      50 mole %                                                                          1,3 Diaminobenzene  50 mole %           (5) 6FDA-DBNDA                                                                           100 mole %                                                                          1,3 Diaminobenzene                                                                            50 mole %                                                                          1,5 Diaminonaphthalene                                                                            50 mole %           (6) 6FDA-3,4-sulfone                                                                     100 mole %                                                                          3,3'Diaminophenylsulfone                                                                      50 mole %                                                                          4,4' Diaminophenylsulfone                                                                         50 mole %           (7) 6FDA-NDA                                                                             100 mole %                                                                          1,5 Diaminonaphthalene                                                                       100 mole %                                    (8) 6FDA-DB                                                                              100 mole %                                                                          1,3 Diaminobenzene                                                                           100 mole %                                    (9) 6FDA-DUR                                                                             100 mole %                                                                          2,3,5,6-Tetramethyl-1,4-                                                                     100 mole %                                                     Phenylenediamine                                             (10) 6FDA-ABN                                                                            100 mole %                                                                          3,5 Diaminobenzonitrile                                                                      100 mole %                                    (11) 6FDA-BENZOATE                                                                       100 mole %                                                                          3,5 Diamino-Methylbenzoate                                                                   100 mole %                                    (12) 6FDA-6F44                                                                           100 mole %                                                                          4,4'-Diphenylhexafluoroisopro-                                                               100 mole %                                                     pylidenediamine                                              (13) 6FDA-6F33                                                                           100 mole %                                                                          3,3'-Diphenylhexafluorolsopropyli-                                                           100 mole %                                                     denediamine                                                  __________________________________________________________________________

                                      TABLE 4                                     __________________________________________________________________________    CHEMICAL FORMULAE OF EXAMPLES 15 TO 19                                        EXAMPLE-NAME FORMULA                                                          __________________________________________________________________________    (15) CELLULOSETRIACETATE                                                                    ##STR12##                                                       (16) HEXAFLUOROPOLY- SULFONE                                                                ##STR13##                                                       (17) PES                                                                                    ##STR14##                                                       (18) TETRAMETHYL- METHYLENE- POLYSULFONE                                                    ##STR15##                                                       (19) PTMSP                                                                                  ##STR16##                                                       __________________________________________________________________________

Forming Film of Memebrane Forming Polymer on the Ultem® AsymmetricPorous Support

The membrane forming polymer solution was deposited through a precisionmedical injection pump first on a stainless steel ramp and then allowedto flow down onto a water bath having two motor driven endless rollerbelts inside the walls of the bath which moved the water in a directionaway from the ramp to a point downstream where it overflowed at the endof the bath. The temperature of the water was set between 10 to 25° C.(preferably 15 to 18° C.) and the water overflow rate was also adjustedbetween 1.5 to 2.0 gallons per minute (GPM). The solution spread rapidlyover the surface of the water as the casting solvent evaporated to forman ultra thin film. After allowing the ultrathin film to travel between30 to 60 cm away from the ramp, the film was continuously drawn from thesurface of the water by the porous Ultem® asymmetric support. Thecomposite membrane (membrane forming polymer on Ultem® support) wasimmediately dried an oven at a temperature between 60 to 70° C. toremove the water trapped between the ultrathin film and the asymmetricporous support and all the residual solvents remaining in the ultrathinfilm. These steps were performed in a class 10,000 clean room.

Preparation of the Thermally Cured Silicone

A platinum-complex catalyst PC072 was diluted to a 1% solution by weightin heptane. A mixture of Sylgard® 184 (5 parts), PS123 (30-35%)methylhydro (65-70%) dimethylsiloxane copolymer crosslinker (1 part),heptane (69 parts), and the diluted PC072 (0.002 parts) was placed in asealed bottle and tumbled for approximately 20 minutes. The resultingpartially cured silicone polymer solution was filtered and degassedbefore use.

Coating of the Thermally Curable Silicone to Form the Sealing Layer

The partially cured silicone polymer prepared as shown above wassolution coated onto the composite membrane by a meniscus coatingprocess at a constant speed (preferably between 30 to 150 cm/min). Afterthe solvent (heptane) was all allowed to evaporate in ambient air thecoated composite (siloxane/membrane-forming-polymer/Ultem®/backingcloth) was post-cured in an oven at 65° to 70° C. for 15 to 20 minutesto complete the cross-linking reaction of the silicone. The siliconecoating process was carried out in a class 10,000 clean room.

Preparation of the UV Cured Silicone

The antimony catalyzed UV curable silicone was prepared by mixing 8 wt.parts of GE9315 polydimethylsiloxane resin (available from the GeneralElectric Silicones Division) with 0.16 wt. parts of GE9310C catalyst in92 wt. parts of heptane. The resulting solution was mixed at ambienttemperature for 10 to 15 minutes and filtered before use.

Coating of the Ultraviolet Curable Silicone to Form the Sealing Layer

The antimony catalyzed UV curable silicone solution was solution coatedonto the composite membrane by a meniscus coating process at a constantspeed (preferably between 30 to 150 cm/min). After the solvent (heptane)was evaporated in ambient air the composite membrane(silicone/membrane-forming-polymer/Ultem®-support/backing cloth) wasexposed to a short wave ultraviolet light for two minutes at ambienttemperature to cure the silicone. Finally the UV cured compositemembrane was dried in an oven at 65° to 70° C. for 10 to 15 minutes toremove any residual solvent. This coating process was also performed ina class 10,000 clean room.

Gas Transport Performance of the Composite Membrane

Except in special cases, the gas permeance and selectivity for thecomposite membranes were measured at 50° C. using mixed gases. Thefeed-side pressure was 5170 cm Hg for carbon dioxide/methane, carbondioxide/nitrogen, carbon dioxide/methane/ethane/propane, carbondioxide/methane/hexane, carbon dioxide/methane/butane, carbondioxide/methane/butene, carbon dioxide/methane/m-xylene, carbondioxide/methane/toluene, carbon dioxide/methane/cyclohexane mixtures and1034 cm Hg at ambient temperature for oxygen/nitrogen. The permeate-sidepressure was atmospheric pressure for all measurements.

TABLE 5 summarizes the gas permeance and selectivity for membranes madeaccording to Examples 1-21 under various test conditions. Theproportions of all gas mixtures are given in relative molar amounts.

    __________________________________________________________________________                                    permeance  std. ft.sup.3 /h ·                                        ft.sup.2 at 100 psi!                          Example                                                                            protecting                 (6.1 · 10.sup.-6 cm.sup.3                                            (STP)/cm.sup.2 · sec ·                                      cm Hg)            selectivity                 No.  layer                                                                              feed gas composition  N.sub.2                                                                            CH.sub.4                                                                          CO.sub.2                                                                          O.sub.2                                                                            CO.sub.2 /N.sub.2                                                                 CO.sub.2 /CH.sub.4                                                                 O.sub.2                                                                       /N.sub.2           __________________________________________________________________________     1   GE 9315                                                                            10% CO.sub.2 IN CH.sub.4                                                                            --   0.418                                                                             12.2                                                                              --   --  29.3 --                           10% CO.sub.2 IN N.sub.2                                                                             0.57 --  11.19                                                                             --   19.6                                                                              --   --                           10% CO.sub.2 IN CH.sub.4 + 1060 PPM HEXANE                                                          --   0.240                                                                             7.23                                                                              --   --  30.1 --                           10% CO.sub.2 IN CH.sub.4 + 9800 PPM BUTANE                                                          --   0.239                                                                             6.99                                                                              --   --  29.2 --                      Sylgard                                                                            10% CO.sub.2 IN CH.sub.4                                                                            --   0.293                                                                             8.62                                                                              --   --  29.4 --                      184  10% CO.sub.2 IN CH.sub.4 + 1060 PPM HEXANE                                                          --   0.260                                                                             6.68                                                                              --   --  29.5 --                           10% CO.sub.2 IN CH.sub.4 + 9800 PPM BUTENE                                                          --   0.235                                                                             6.68                                                                              --   --  28.4 --                           10% CO.sub.2 IN CH.sub.4 + 9800 PPM BUTANE                                                          --   0.236                                                                             6.70                                                                              --   --  28.7 --                  2   GE 9315                                                                            10% CO.sub.2 IN CH.sub.4                                                                            --   0.215                                                                             7.11                                                                              --   --  33.3 --                           10% CO.sub.2 IN N.sub.2                                                                             0.658                                                                              --  11.9                                                                              --   18.1                                                                              --   --                           10% CO.sub.2 IN CH.sub.4 + 530 PPM CYCLOHEXANE                                                      --   0.200                                                                             6.65                                                                              --   --  33.3 --                  3   GE 9315                                                                            10% CO.sub.2 IN CH.sub.4                                                                            --   0.570                                                                             12.70                                                                             --   --  22.3 --                           10% CO.sub.2 IN CH.sub.4 + 100 PPM TOLUENE                                                          --   0.329                                                                             7.52                                                                              --   --  22.8 --                           10% CO.sub.2 IN CH.sub.4 + 5530 PPM CYCLOHEXANE                                                     --   0.325                                                                             7.40                                                                              --   --  22.8 --                  4   GE 9315                                                                            10% CO.sub.2 IN CH.sub.4                                                                            --   0.155                                                                             5.36                                                                              --   --  34.5 --                           10% CO.sub.2 IN CH.sub.4 + 1000 PPM HEXANE                                                          --   0.173                                                                             4.39                                                                              --   --  25.4 --                           10% CO.sub.2 IN CH.sub.4 + 9800 PPM BUTENE                                                          --   0.164                                                                             5.04                                                                              --   --  30.8 --                  5   GE 9315                                                                            10% CO.sub.2 IN CH.sub.4                                                                            --   0.541                                                                             11.62                                                                             --   --  21.5 --                           10% CO.sub.2 IN CH.sub.4 + 9800 PPM BUTENE                                                          --   0.615                                                                             11.8                                                                              --   --  19.2 --                           10% CO.sub.2 IN CH.sub.4 + 9800 PPM BUTANE                                                          --   0.529                                                                             10.58                                                                             --   --  20.0 --                  6   GE 9315                                                                            10% CO.sub.2 IN CH.sub.4                                                                            --   0.155                                                                             5.26                                                                              --   --  34.0 --                           10% CO.sub.2 IN CH.sub.4 + 1000 PPM HEXANE                                                          --   0.127                                                                             4.37                                                                              --   --  34.4 --                           10% CO.sub.2 IN CH.sub.4 + 9800 PPM BUTENE                                                          --   0.132                                                                             4.37                                                                              --   --  33.0 --                  7   GE 9315                                                                            10% CO.sub.2 IN CH.sub.4                                                                            --   1.277                                                                             24.00                                                                             --   --  18.8 --                           10% CO.sub.2 IN N.sub.2                                                                             1.419                                                                              --  22.74                                                                             --   16.0                                                                              --   --                           21% O.sub.2 IN 79% N.sub.2.sup.1                                                                    2.47 --  --  10.8 --  --   4.38                8   GE 9315                                                                            10% CO.sub.2 IN CH.sub.4                                                                            --   0.267                                                                             7.71                                                                              --   --  28.9 --                           10% CO.sub.2 IN CH.sub.4 + 1000 PPM HEXANE                                                          --   0.221                                                                             6.44                                                                              --   --  29.1 --                           10% CO.sub.2 IN CH.sub.4 + 5% PROPANE + 9.7%                                                        --   0.170                                                                             5.57                                                                              --   --  32.7 --                           ETHANE                                                               9   GE 9315                                                                            10% CO.sub.2 IN CH.sub.4                                                                            --   1.213                                                                             18.80                                                                             --   --  15.5 --                           10% CO.sub.2 IN CH.sub.4 + 100 PPM TOLUENE                                                          --   0.856                                                                             13.69                                                                             --   --  16.0 --                           10% CO.sub.2 IN CH.sub.4 + 5% PROPANE + 9.7%                                                        --   1.644                                                                             24.45                                                                             --   --  14.9 --                           ETHANE                                                                        21% O.sub.2 IN 79% N.sub.2.sup.1                                                                    3.33 --  --  12.1 --  --   3.62               10   GE 9315                                                                            10% CO.sub.2 IN CH.sub.4                                                                            --   0.442                                                                             10.94                                                                             --   --  24.8 --                           21% O.sub.2 IN 79% N.sub.2.sup.1                                                                    0.452                                                                              --  --   3.25                                                                              --  --   7.15               11   GE 9315                                                                            10% CO.sub.2 IN CH.sub.4                                                                            --   0.825                                                                             19.32                                                                             --   --   23.42                                                                             --                 12   GE 9315                                                                            10% CO.sub.2 IN CH.sub.4                                                                            --   0.498                                                                             10.15                                                                             --   --  20.4 --                           10% CO.sub.2 IN CH.sub.4 + 40 PPM M-XYLENE                                                          --   0.466                                                                             9.42                                                                              --   --  20.2 --                           10% CO.sub.2 IN CH.sub.4 530 PPM CYCLOHEXANE                                                        --   0.423                                                                             8.71                                                                              --   --  20.6                    13   GE 9315                                                                            10% CO.sub.2 IN CH.sub.4                                                                            --   0.439                                                                             12.94                                                                             --   --  29.5 --                 14   GE 9315                                                                            10% CO.sub.2 IN CH.sub.4                                                                            --   0.185                                                                             3.84                                                                              --   --  20.8 --                           10% CO.sub.2 IN CH.sub.4 + 40 PPM M-XYLENE                                                          --   0.110                                                                             2.36                                                                              --   --  21.5 --                           10% CO.sub.2 IN CH.sub.4 + 530 PPM CYCLOHEXANE                                                      --   0.115                                                                             2.41                                                                              --   --  20.9 --                 15   GE 9315                                                                            10% CO.sub.2 IN CH.sub.4                                                                            --   0.716                                                                             12.24                                                                             --   --  17.1 --                           10% CO.sub.2 IN N.sub.2                                                                             0.586                                                                              --  8.83                                                                              --   15.1                                                                              --   --                           49% CO.sub.2 IN CH.sub.4                                                                            --   0.910                                                                             13.2                                                                              --   --  14.5 --                           10% CO.sub.2 + 40 PPM XYLENE                                                                        --   0.780                                                                             12.25                                                                             --   --  15.7 --                           10% CO.sub.2 + 40 PPM TOLUENE                                                                       --   0.735                                                                             11.98                                                                             --   --  16.3 --                           9.78% CO.sub.2 + 4.82 PROPANE + 9.74%                                                               --   0.687                                                                             12.71                                                                             --   --  18.5 --                           ETHANE IN CH.sub.4                                                            10% CO.sub.2 + 530 PPM CYCLOHEXANE                                                                  --   0.667                                                                             11.07                                                                             --   --  16.6 --                           10% CO.sub.2 IN CH.sub.4 + 1060 PPM HEXANE                                                          --   0.723                                                                             12.08                                                                             --   --  16.7 --                           10% CO.sub.2 IN CH.sub.4 + 9800 PPM BUTENE                                                          --   0.764                                                                             12.45                                                                             --   --  16.2 --                           10% CO.sub.2 IN CH.sub.4 + 9800 PPM BUTANE                                                          --   0.800                                                                             12.80                                                                             --   --  16.0 0                  16   GE 9315                                                                            10% CO.sub.2 IN CH.sub.4                                                                            --   0.468                                                                             7.20                                                                              --   --  15.4 --                           10% CO.sub.2 IN N.sub.2                                                                             0.391                                                                              --  7.03                                                                              --   17.9                                                                              --   --                 17   GE 9315                                                                            10% CO.sub.2 IN CH.sub.4                                                                            --   0.094                                                                             0.77                                                                              --   --  8.15 --                 18   GE 9315                                                                            10% CO.sub.2 IN CH.sub.4                                                                            --   0.126                                                                             2.17                                                                              --   --  17.2 --                 19   GE 9315                                                                            10% CO.sub.2 IN CH.sub.4   0.870                                                                             4.89          5.62                   .sup. 20.sup.2                                                                     GE 9315                                                                            10% CO.sub.2 IN CH.sub.4  blend ratio DBS:NDA                                                       --:10!                                                                             0.235                                                                             7.17                                                                              --   --  30.5 --                           10% CO.sub.2 IN CH.sub.4  blend ratio DBS:NDA                                                       --:25!                                                                             0.287                                                                             8.47                                                                              --   --  29.5 --                           10% CO.sub.2 IN CH.sub.4  blend ratio DBS:NDA                                                       --:50!                                                                             0.725                                                                             15.41                                                                             --   --  21.3 --                 .sup. 21.sup.3                                                                     Sylgard                                                                            10% CO.sub.2 IN CH.sub.4                                                                            --   0.332                                                                             9.37                                                                              --   --  28.2 --                      184                                                                      __________________________________________________________________________     .sup.1 Pressure 200 psig ambient temperature                                  .sup.2 In this experiment a membrane was made by the same procedures as       were employed in examples 1-14 except that two 6FDA type polyimide            polymers were blended together                                                .sup.3 Composite membrane having two 6FDA type polyimide polymer layers       superimposed; 6FDADUR (adjacent to asymmetric poroussupport) and 6FDADBS 

The trials carried out in the presence of impurities (including hexane,butane, butene, cyclohexane, toluene, propane, ethane and m-xylene)confirm that the composite gas separation membranes of the invention cantolerate moderate concentrations of such impurities while still yieldinggood performance in terms of both permeance and selectivity.

While several embodiments of the invention have been illustrated anddescribed above, it is not intended to be limited to the details shown,since various modifications and structural changes may be made withoutdeparting from the spirit of the present invention, which is defined bythe claims below.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims:
 1. A composite membrane comprising a porous asymmetric support having a first and a second surface area, said first surface area contacting an ultrathin polymer layer with a thickness less than 100 nm, said first surface area having an average pore diameter of about 5 nm to about 20 nm, and said second surface area having a pore diameter greater than that of the first surface area.
 2. A composite membrane according to claim 1, wherein the first surface area has about 10¹⁰ to about 10¹¹ pores/cm² and the second surface area has fewer pores/cm than the first surface area.
 3. A composite membrane according to claim 1, wherein the thickness of the ultrathin polymer layer is <40 nm.
 4. A composite membrane according to claim 3 wherein the thickness is 20-30 nm.
 5. A composite membrane according to claim 1, wherein the polymer of the ultrathin layer is a membrane forming polymer being selected from the group consisting of cellulose triacetate (CTA), hexafluoropolysulfone (6F-PS), polytrimethylsilylpropyne (PTMSP), polysulfone (PS), polyetherimide (PEI), polypropylene oxide (PPO) and mixtures thereof.
 6. A composite membrane according to claim 1 wherein the asymmetric porous support is a polyetherimide.
 7. A composite membrane according to claim 6 wherein the polyetherimide is a reaction product of at least one phenoxy-phenyldicarboxylic acid anhydride and at least one phenylenediamine.
 8. A composite membrane according to claim 1 further comprising a porous structural support which is in contact with the asymmetric porous support.
 9. A composite membrane according to claim 8 in which the porous structural support is a woven fabric.
 10. A process for making a composite gas separation membrane comprising the following steps:(A) Providing a base support having a first planar surface; (B) Forming on said first planar surface an asymmetric porous support, such asymmetric porous support comprising mutually coplanar first and second regions, the thickness of said first region being substantially greater than the thickness of said second region, said first region having a microporous structure and being placed adjacent to said first planar surface, said second region having a high density of pores having an average diameter of less than about 1,000 Ångstroms, said second region comprising a first surface integrally connected to said first region and a second surface; (C) Preparing a dissolved polymer solution comprising (1) at least one 6FDA-type polyimide polymer having been formed by the dehydration of the condensation reaction product of at least one aromatic dianhydride and at least one aromatic diamine; and (2) a solvent for the polyimide polymer which is immiscible with a selected liquid, said selected liquid being a nonsolvent for the polyimide polymer and having a higher surface tension than said dissolved polymer solution; (D) Providing a bath containing a pool of said selected liquid, said pool having a smooth surface with high surface tension; means for depositing the dissolved polymer solution onto said smooth surface at a deposition point; and means for conveying the smooth surface away from the deposition point; (E) Depositing said dissolved polymer solution onto said smooth surface, and removing the solvent from the dissolved polymer solution to yield an ultrathin 6-FDA type polyimide polymer selective membrane film having a leading edge, being the farthest-most edge of said polymer film floating away from said deposition point on said smooth surface, which leading edge is conveyed by the smooth surface away from the deposition point; (F) Conveying said asymmetric porous support into a position so that said second surface is adjacent to said leading edge; and (G) Picking up said leading edge onto said second surface and conveying said second surface tangentially away from said smooth surface; yielding a composite membrane comprising an ultrathin 6FDA-type polyimide polymer selective membrane film.
 11. The process of claim 10 comprising the following additional steps:(H) Preparing a solution comprising a sealing polymer and a sealing polymer solvent; (I) Depositing a layer of such solution onto said ultrathin 6FDA-type polyimide polymer selective membrane film; and (J) Removing said sealing polymer solvent and curing said polymer, yielding a composite membrane comprising a nonporous protective sealing polymer layer.
 12. The process of claim 11 in which the sealing polymer is a polydimethylsiloxane.
 13. A composite gas separation membrane made by the process of claim
 11. 14. The process of claim 10 in which said base support is a woven fabric and the first planar surface is highly uniform and smooth.
 15. The process of claim 10 in which the asymmetric porous support is stripped away from the base support subsequent to step (B).
 16. The process of claim 15 in which the base support is a web comprising laminated layers of metal and plastic.
 17. The process of claim 15 in which the base support is a glass plate.
 18. A composite gas separation membrane made by the process of claim
 15. 19. The process of claim 10 in which the asymmetric porous support is formed on said first planar surface by solvent cast phase inversion.
 20. A composite gas separation membrane made by the process of claim
 19. 21. The process of claim 10 in which the asymmetric porous support is made of a polyetherimide polymer.
 22. A composite gas separation membrane made by the process of claim
 21. 23. The process of claim 10 in which each aromatic dianhydride is selected from the group consisting of: 5,5'-2,2,2-trifluoro-1-(trifluoromethyl)ethyl-idene-bis-1,3-isobenzofuranedione; 3,4,3',4'-diphenyltetracarboxylic dianhydride; 1,2,4,5-benzene-tetracarboxylic dianhydride; 3,3',4,4'-benzophenonetetracarboxylic-dianhydride; pyromellitic dianhydride; and mixtures.
 24. The process of claim 23 in which each 6FDA-type polyimide polymer is derived from the condensation of a mixture of reagents comprising 5,5'-2,2,2-trifluoro-1-(trifluoromethyl)ethylidene-bis-1,3-isobenzofuranedione with at least one aromatic diamine.
 25. A composite gas separation membrane made by the process of claim
 24. 26. The process of claim 10 in which each aromatic diamine is selected from the group consisting of:2,4,6-trimethyl-1,3-phenylene-diamine;4,4'- 1,4-phenylenebis(1-methyl-ethylidene)!bisaniline; 2,2-bis 4-(4-aminophenoxy)-phenyl!propane; 2,7-bis(4-aminophenoxy)-naphthalene; 4,4'-methyl-ene-bis(2,6-diisopropylaniline); 1,4-bis(4-aminophenoxy)benzene;4,4'-bis(4-aminophenoxy)-biphenyl; 1,3-bis(4-aminophenoxy)benzene; and 4,4'-(methylethylidene)bisaniline; 4-isopropyl-1,3-diaminobenzene; 4,4'-diaminodiphenylether; metaphenylenediamine; paraphenylenediamine; N,N'-metaphenylenebis(m-aminobenzanilide); and 3,3'-diaminobenz-anilide; 2-carboxy-3,5-diaminotoluene; 3,3'-diphenylhexafluoroisopropylidene diamine; 4,4'-diphenylhexafluoroisopropylidene diamine; 1,3-diaminobenzene; 2,3,5,6-tetramethyl-1,4-phenylenediamine; 1,5-diaminonaphthalene; 3,5-diaminobenzonitrile; 3,5-diaminomethylbenzoate; 3,3'-diaminophenylsulfone; 4,4'-diaminophenylsulfone; 3,7-diamino-2,8-dimethyldiphenylenesulfone; and mixtures.
 27. The process of claim 26 in which each aromatic diamine is selected from the group consisting of: 3,3'-diphenylhexafluoroisopropylidene diamine;4,4'-diphenylhexafluoroisopropylidene diamine; 1,3-diaminobenzene; 2,3,5,6-tetramethyl-1,4-phenylenediamine; 1,5-diaminonaphthalene; 3,5-diaminobenzonitrile; 3,5-diaminomethylbenzoate; 3,3'diaminophenylsulfone; 44'-diaminophenylsulfone; 3,7-diamino-2,8-dimethyldiphenylenesulfone; and mixtures.
 28. A composite gas separation membrane made by the process of claim
 27. 29. The process of claim 10 in which the ultrathin 6FDA-type polyimide polymer selective membrane film has a thickness of less than about 1,000 Ångstroms.
 30. The process of claim 29 in which the ultrathin 6FDA-type polyimide polymer selective membrane film has a thickness of less than about 400 Ångstroms.
 31. The process of claim 30 in which the ultrathin 6FDA-type polyimide polymer selective membrane film has a thickness of between about 200 and about 300 Ångstroms.
 32. A composite gas separation membrane made by the process of claim
 29. 33. The process of claim 10 in which each 6FDA-type polyimide polymer is derived from the condensation of a mixture of reagents comprising 3,3',4,4'-benzophenone-tetracarboxylic dianhydride and 5(6)-amino-1-(4'-aminophenyl)-1,3,3-trimethylindane.
 34. A composite gas separation membrane made by the process of claim
 10. 35. A gas separation process comprising:A. providing a gas separation membrane according to claim 34, having feed and permeate sides; B. contacting the membrane with a mixed feed gas composition under pressure; C. collecting separated permeate gas from the permeate side of the membrane; and D. collecting retentate gas from the feed side of the membrane.
 36. The process of claim 35, in which the mixed feed gas composition comprises two or more members of the group consisting of methane, nitrogen, carbon dioxide, hydrogen, oxygen, carbon monoxide, hydrogen sulfide, water, helium, propane, and ethane.
 37. The process of claim 36, in which the mixed feed gas composition comprises a mixture of gases selected from the group of mixtures consisting of: air; nitrogen and oxygen; nitrogen and carbon dioxide; hydrogen and at least one hydrocarbon; carbon monoxide and hydrogen; carbon dioxide and at least one hydrocarbon; hydrogen sulfide and at least one hydrocarbon; hydrogen and nitrogen; helium and nitrogen; and helium and at least one hydrocarbon. 